This proposal requests NIH support for a Symposium to be held in Vancouver 20 June 1990. The topic will be the use of photosensitizing agents for tumor eradication. This will be held in conjunction with the 1990 meeting of the American Society for Photobiology which will provide space and assist with arrangements. Matching funds will be provided by QLT, a Vancouver-based organization engaged in research relating to tumor photosensitization. After administration, the porphyrin product HDP localizes at tumor loci in vivo. Neoplastic lesions can be identified by the resulting fluorescence, and can be selectively destroyed, via porphyrin-catalyzed photo-oxidation, upon irradiation with red light. More recently, other dyes have been developed for this purpose. These show improved properties with regard to action spectrum and localization, and toxicity trials are underway. Periodic conferences are useful for exchange of information among workers in the field, and for providing updates of pre-clinical and clinical results for the benefit of individuals who are beginning research on photosensitization phenomena. During the 1990 meeting, it is proposed that a mix of new and old workers in the field be invited to discuss pre-clinical aspects of photodynamic therapy. QLT Workshop is scheduled 12 months after the June 1989 meeting in Boston. The format of the Symposium will include a 'school' lecture on Light Delivery Systems by Dan Doiron, and a 'general-interest' lecture by Tom Dougherty or Chuck Gomer. The other talks will be scheduled so as to provide ample discussion time. Recordings of all discussion will be made available for participants and others.